1. Field of the Invention
The invention relates to control devices for a low-pressure fluorescent lamp.
2. Discussion of the Related Art
Fluorescent lamps contain gases (neon, argon) at low pressure. The electrical behavior of a fluorescent lamp is similar to that of a zener (avalanche) diode with a resistance in the gas that may become very low and negative after breakdown. Ions moving at high speeds lead the atoms of the gas to assume excited states in which they give out luminous lines.
A control device for the lamp is typically needed, comprising a current source. However, to avoid a migration of ions, the current discharges applied between two electrodes of the lamp should pass in one direction and then in the other, alternately. The practice has been to use an inductor as a discharge control device but the development of electronics has led to the use of control devices typically comprising two electronic switches based on power transistors supplied with DC high voltage and a current transformer to control these transistor-based control devices. A resonant circuit comprising an inductor and a capacitor applies an AC current to the fluorescent lamp. According to the prior art, the transformer is a saturation transformer that limits the current in the lamp by saturation of its magnetic core and leads to the switching of the switches of the control devices. The electronic switches generally use bipolar technology power transistors for the switching and parallel and reverse-connected diodes to let through the current during the alternations, and various protection elements such as diodes and capacitors.
These transformer devices are very bulky and costly because they require many components and allow only a very low degree of integration. Furthermore, the storage time of the bipolar transistors is a highly variable characteristic, for example ranging from 2 to 7 microseconds. This variation is not negligible as compared with the time at the end of which the transformer gets saturated for a current half-wave: it is about three microseconds for an alternation time of about ten microseconds. Hence, the time at the end of which the bipolar transistor goes off after saturation of the transformer in an alternation varies from 5 to 10 microseconds. This is very troublesome. In practice, the storage time of each transistor is measured at the end of its manufacture in order it may be classified in a group corresponding to a narrow range of values for use in a control device matched by means of resistors with this range of values. All this entails heavy penalties and is very costly.